In manufacturing semiconductor integrated circuits, a semiconductor wafer made of a silicon substrate is generally subjected to various processes, e.g., film formation, etching, oxidation, diffusion, modification and native oxide film removal. These processes are carried out by a single wafer processing apparatus which processes wafers one-by-one or by a batch type processing apparatus, which processes a plurality of wafers simultaneously. When performing the processes using a vertical batch type processing apparatus, for example, the wafers are first loaded from a cassette accommodating therein a plurality of wafers, e.g., 25 wafers, into a vertical wafer boat, and then supported by the boat in multi-levels.
The number of wafers loaded into the wafer boat depends on, for example, the size of the wafers, but about 30 to 150 wafers may be loaded onto the wafer boat. The wafer boat is loaded into a gas-evacuable processing chamber from the bottom, and then the interior of the processing chamber is kept airtight. Thereafter, a heat treatment is performed while controlling various processing conditions such as flow rates of processing gases, a processing pressure and a processing temperature.
From among several factors improving characteristics of semiconductor integrated circuits, it is important to improve the characteristics of insulating films in semiconductor integrated circuits. SiO2, PSG (Phospho Silicate Glass), P—SiO (Plasma SiO), P—SiN (Plasma SiN), SOG (Spin On Glass) and Si3N4 (silicon nitride film) are used as the insulating films in semiconductor integrated circuits. Particularly, silicon nitride films tends to be broadly used since they have an insulating property relatively better than silicon oxide films and serves sufficiently well as etching stopper films or interlayer insulating films.
Recently, demand for lower dielectric constants (low-k) and an improvement of an etching resistance has increased in order to improve characteristics of circuit elements. In such circumstances, since a desired process using a vertical batch-type film forming apparatus can be performed on a wafer without exposing the wafer to a high temperature, a film forming method, which forms a film by repeatedly depositing one or several layers at an atomic level or one or several layers at a molecular level while intermittently supplying source gases, has been suggested. Such a film forming method is called as an atomic layer deposition (ALD) method.
In a conventional ALD method, a silicon nitride (SiN) film is formed by using a dichlorosilane (hereinafter, referred to as “DCS”) gas as a silicon-containing source gas and a NH3 gas as a nitriding gas. Specifically, the DCS gas and the NH3 gas are alternately and intermittently supplied into a processing chamber and an RF (high frequency) power is applied when supplying the NH3 gas to thereby generate a plasma and accelerate a nitridation process.
Alternatively, the NH3 gas can be activated by using heat without generating the plasma. As described above, one or several layers of the DCS at a molecular level are adsorbed on a wafer surface by supplying the DCS gas into the processing chamber, and a residual DCS gas is purged by an inert gas or discharged by vacuum-evacuation. Thereafter, a nitride film is formed by supplying the NH3 gas and accelerating a nitridation process at a low temperature. A series of these procedures are repeatedly performed.
There have also been known several methods to meet a recent demand for increasing a film forming rate or a concentration of silicon in a film. In one method, a buffer tank with a certain capacity is installed in a supply passage of a source gas and a large amount of source gas is temporarily stored in the buffer tank and discharged from the buffer tank when supplying a processing gas, so that the large amount of source gas is intermittently supplied into the processing chamber. In another method, a first tank for charging the source gas is installed at the downstream side of the supply passage of the source gas and a second tank for charging a pressurized N2 gas is installed at the upstream side of the supply passage of the source gas, so that a moving velocity of the source gas is accelerated by the pressurized N2 gas and a gas partial pressure of the source gas in the processing chamber increases.
The large amount of source gas can be supplied in a pulse-like manner and within a short time by installing the buffer tank in the supply passage of the source gas as described above, which results in an increase in the film forming rate. However, supplying the large amount of source gas as described above may cause a problem in that an in-plane uniformity of a silicon nitride film on a wafer surface is deteriorated. This problem cannot be solved by increasing the gas partial pressure of the source gas in the processing chamber as described above.